The G2X uses the same Wolfson DAC as the Galaxy S. As does the original Atrix.
Quite a few Android phones now have good audio quality, as manufacturers realise it's something people want in the top handsets.
My Sony Xperia S sounds better than my old Creative Zen Vision:M, especially with Neutron Music Player or Poweramp. I noticed a marked improvement in detail when I started using the Xperia. In fact, the audio output drives my Hifiman RE-262 earphones, which are said to 'need' an amp, very well (there's a slight improvement with an amp, but I mean slight).
So yes, it is worth getting decent headphones for the best audio experience.
Even on my cheap Samsung Galaxy Pocket which is a fairly new basic model (which seems to focus more towards audio as it has some extra features regarding that) I was postively surpised how great it sounds and the imaging and soundstage sounded much better than I had expected. Drives even the M-Audio Q40 64 ohm headphones to very satisfactory volume levels at like 80~100% depending on settings (pretty much same vol levels I'd use with amp on the comp) and the bass response is suprisingly strong but also textured, punchy and well controlled and I've always thought this headphone almost demands an amp to sound great. I seem to prefer the default player to PowerAMP though, the default player seems to sound better to my ears, more neutral. Seems to have better synergy than with Realtek onboard ALC889 chip.
Thank you, I actually didn't know this. I always was pleasantly surprised by the audio quality I got out of it. Now I know why.